Conventionally, a valve timing control apparatus includes a drive rotational member synchronously rotated with a crankshaft, a driven rotational member provided coaxially with the drive rotational member and rotated with a camshaft, a fluid pressure chamber provided in at least one of the drive rotational member and the driven rotational member, a vane dividing the fluid pressure chamber into an advanced angle chamber and a retarded angle chamber, and a relative rotational phase-controlling mechanism for supplying or discharging a working fluid to or from one or both of the advanced angle chamber and the retarded angle chamber for changing a relative position of the vane to the fluid pressure chamber and for controlling a relative rotational phase between the drive rotational member and the driven rotational member within a range from a most retarded angle phase at which a volume of the retarded angle chamber becomes maximum and a most advanced angle phase at which a volume of the advanced angle chamber becomes maximum.
Further, a biasing mechanism (for example, a torsion spring) is provided between the drive rotational member and the driven rotational member for biasing the relative rotational phase between the rotational members toward the maximum advanced angle phase.
Further, a locking mechanism is provided for restraining the relative rotational phase between the drive rotational member and the driven rotational member so as to start an engine at an optimum condition.
In the locking mechanism, for example, for making a state of lock, a locking member provided at the drive rotational member is biased toward the driven rotational member by means of spring, and the locking member is inserted into a locking fluid chamber provided at the driven rotational member. Thus, the relative rotation is restrained. For releasing the state of lock, a locking fluid is supplied into the locking fluid chamber to increase a fluid pressure, and the locking member is pulled back toward the drive rotational member.
In a conventional valve timing control apparatus including the control mechanism for the relative rotational phase, the biasing mechanism, and the locking mechanism, a torque of the biasing means is set on the basis of an average torque of the camshaft. In other words, according to a first conventional technique (for example, described in US2001/0039933A), a minimum of the torque of the biasing mechanism is set to 10% of an average torque within an idling rotational range of the camshaft, and a maximum of the torque of the biasing mechanism is set to an average torque of the camshaft rotating under its own inertia. Further, according to a second conventional technique (for example, described in U.S. Pat. No. 6,155,219A), the maximum is set to an average inertia torque of the camshaft within a period until the spark ignition occurs after-one cycle of rotation of the crankshaft at the start time of the combustion engine.
Recently, in order not only to obtain smooth start of an engine, but also to obtain an adjustable range of the relative rotational phase between the rotational members both in the advanced angle and in the retarded angle, a valve timing control apparatus is proposed in which a lock phase, at which a locking mechanism inhibits the relative rotation between the rotational members, is provided in an intermediate phase between the most retarded angle phase and the most advanced angle phase.
Further, a similar kind of a valve timing control apparatus having an intermediate lock structure is proposed in which the relative rotational phase is restricted from going back toward the retarded angle at a single step or plural steps, the relative rotational phase is sequentially stepped up toward the intermediate phase, and thus an intermediate lock is rapidly realized.
In view of the lock phase, in the first conventional technique and the second conventional technique, the lock phase is not set to the intermediate phase. In other words, in the apparatus described in the first conventional technique, as described in a paragraph [0028] and FIG. 2 in JP2000-179314A (US2001/0039933A), the lock phase is set to the most retarded angle phase. In contrast, in the apparatus described in the second conventional technique, as described in a paragraph [0025] and FIG. 2 in JP2000-145415A (U.S. Pat. No. 6,155,219A), the lock phase is set to the most advanced angle phase.
As described above, in a field of the valve timing control apparatus of the intermediate locking structure, a technique for setting a torque of the biasing mechanism is not sufficiently established. Accordingly, a torque has been relatively roughly set for the biasing mechanism.
In a valve timing control apparatus having a lock phase (so called an intermediate phase) at which a locking mechanism functions, a need thus exists for a valve timing control apparatus in which a torque generated by a biasing mechanism can be set without excess or deficiency, a relative rotational phase can be easily controlled, and an intermediate lock can be realized with reliability. Further, a need thus exists for a method for setting a torque of a biasing mechanism enabling to realize such apparatus. The present invention has been made in view of the above circumstances and provides such a valve timing control apparatus and a method for setting a torque of a biasing mechanism.